Nobody is 100 per cent certain why humans yawn, but it definitely is contagious: in one experiment half of the people yawned while watching a video of others yawning. A common theory is that the influx of oxygen from a good yawn revives us by getting rid of excess carbon dioxide in our blood – although some research is now refuting this idea. Instead, we may yawn simply to stretch our tongue and throat muscles, as well as to communicate boredom or tiredness to our peers. One recent study even suggested that yawning may help to regulate brain temperature by exposing the roof of the mouth to cooler air – a bit like a heat sink.